China is remaining silent about an accusation from Japan that one of its nuclear submarines intruded into Japanese waters this week.

Official state media has not reported the Japanese government's charge and demand for an apology.

Instead, China's foreign ministry repeated Saturday a comment made Friday that Beijing is investigating the matter.

Japan mobilized its navy Wednesday after a submarine was spotted near the southern island of Okinawa. At least one surveillance airplane and two destroyers spent the next two days tracking the vessel in the open sea.

Japan said sounds made by its propeller were that of a Chinese military nuclear-powered submarine.